Hitherto, there has been proposed an automatic transmission device capable of establishing nine forward speeds and a reverse speed with four planetary gear mechanisms, three clutches, and three brakes (see Patent Document 1, for example). The configuration of the device is illustrated in FIG. 7. As illustrated in the drawing, an automatic transmission device 901 according to a conventional example as the background art includes single-pinion first to fourth planetary gear mechanisms 910, 920, 930, and 940 each composed of a sun gear 911, 921, 931, or 941 which is an externally toothed gear, a ring gear 913, 923, 933, or 943 which is an internally toothed gear, and a carrier 912, 922, 932, and 942 that rotatably and revolvably holds a plurality of pinion gears 914, 924, 934, and 944 by coupling the pinion gears 914, 924, 934, and 944 to each other, respectively. The sun gear 911 and the sun gear 921 are coupled to each other by a first coupling element 951. The ring gear 913 and the carrier 922 are coupled to each other by a second coupling element 952. The ring gear 923, the carrier 932, and the carrier 942 are coupled to each other by a third coupling element 953. The fourth planetary gear mechanism 940 is formed on the outer peripheral side of the third planetary gear mechanism 930. The ring gear 933 and the sun gear 941 are coupled to each other by a fourth coupling element 954. The sun gear 931 is connected to an input shaft 903 via a clutch C901, and connected to a case 902 via a brake B901. The second coupling element 952 is connected to the input shaft 903 via a clutch C902. Further, the fourth coupling element 954 is connected to the input shaft 903 via a dog clutch DC. The first coupling element 951 is connected to the case 902 via a dog brake DB. The ring gear 943 of the fourth planetary gear mechanism 940 is connected to the case 902 via a brake B902. An output gear 904 is connected to the carrier 912 of the first planetary gear mechanism 910.
In the automatic transmission device 901 according to the conventional example, the gear ratios λ1, λ2, λ3, and λ4 of the first to fourth planetary gear mechanisms 910, 920, 930, and 940 (the number of teeth of the sun gear/the number of teeth of the ring gear in each of the planetary gear mechanisms) are set to 0.36, 0.36, 0.56, and 0.66, respectively. As illustrated in the operation table of FIG. 8, a first forward speed to a ninth forward speed and a reverse speed are established, and the gear ratio width which is calculated as the gear ratio of the first forward speed (lowest speed)/the gear ratio of the ninth forward speed (highest speed) is 10.02.
With the ninth forward speed which is the highest speed established, the clutch C901, the clutch C902, and the brake B902 are engaged, and the dog clutch DC, the dog brake DB, and the brake B901 are disengaged. Thus, all (four) of the first to fourth planetary gear mechanisms 910, 920, 930, and 940 operate as a gear mechanism for torque transfer from the input shaft 903 to the output gear 904. With the eighth forward speed which is one step lower than the highest speed established, meanwhile, the clutch C902, the brake B901, and the brake B902 are engaged, and the clutch C901, the dog clutch DC, and the dog brake DB are disengaged. Thus, two of the planetary gear mechanisms, namely the first planetary gear mechanism 910 and the second planetary gear mechanism 920, operate as a gear mechanism for torque transfer from the input shaft 903 to the output gear 904.